1. Field of the Invention
The present invention relates generally to the field of optical communications and in particular to a device and method for providing optical signal amplification in the longer wavelength or tail region of a given gain spectrum.
2. Technical Background
Rare earth doped optical amplifiers and particularly erbium-doped fiber amplifiers (EDFAs) are used extensively and almost exclusively to amplify optical signals in today""s communications systems and networks. The well-known benefits of rare earth doped optical amplifiers include cost effectiveness, good noise performance, a relatively broad polarization insensitive bandwidth, low insertion loss, and improved crosstalk performance over other amplifier technologies. EDFAs are increasingly being used in wavelength division multiplexed (WDM) optical communications systems and networks.
As service providers strive to keep up with the ever-growing demand for capacity, attention has been focused on providing as many WDM optical channels as possible within a given WDM system. As such, broadband optical amplifiers are being developed to realize dense WDM (DWDM) optical systems and networks.
The total gain spectrum for an EDFA is very wide, as shown in FIG. 2. The usable gain spectrum extends from around 1525 nm to 1565 nm and this is conventionally referred to as the erbium C-band. With appropriate gain equalization, an approximately 40 nm bandwidth is provided for DWDM applications. FIG. 2 also shows that the gain for an EDFA drops sharply in the spectral region below 1525 nm and the spectral region above 1565 nm. Although conventional gain equalization techniques cannot be practically implemented to further increase the gain bandwidth of EDFAs, the demand for higher capacity lightwave systems has renewed the interest in signal amplification in the longer wavelength range between about 1565 nm and 1620 nm, commonly referred to as the L-band or extended band. See, for example, Massicott et al., xe2x80x9cLow noise operation of ER3+ doped silica fiber amplifier around 1.6 micron,xe2x80x9d Elec. Lett., Volume 26, Number 20, pp 1645-1646, September 1990. In spite of the appreciation of the potential use of the long wavelength tail of the erbium gain window for optical signal amplification, little attention is evidenced in the public literature to the optimization of L-band amplifiers.
The performance of an L-band amplifier is limited by at least three inter-related factors. These include a) a reduced gain coefficient in the band of interest, b) self-saturation by short wavelength amplified spontaneous emission (ASE), and c) background loss in the long fiber coils necessary for high gain operation. The intrinsic reduction in gain/loss ratio for an L-band amplifier over a C-band amplifier results in reduced power conversion efficiency. This is further exaggerated by the reduction in average inversion which accompanies self-saturation and which reduces the already low gain coefficient even further, resulting in even more length dependent efficiency reduction. Furthermore, if the first stage of a multistage amplifier is operated at low inversion, the noise performance of the amplifier is significantly compromised. However, operation at high inversion produces C-band ASE which will reduce the power conversion efficiency of the amplifier.
Accordingly, the inventors have recognized a need to improve the performance of an L-band amplifier and more specifically have targeted the tradeoff between noise figure and power conversion efficiency to address this.
An embodiment of the present invention is directed to an optical amplifier for amplifying optical signals in a longer wavelength, tail region of a gain spectrum associated with the amplifier, including a rare earth-doped gain medium referred to as a gain stage of the amplifier; a source of pump power connected to the gain medium; and a filter distributed over the gain stage, wherein the filter attenuates light associated with amplified spontaneous emission (ASE) in the amplifier, such that substantially only the optical signals in the longer wavelength region of the gain spectrum are amplified.
Another embodiment of the invention is directed to an optical amplifier for amplifying optical signals in a longer wavelength, tail region of a gain spectrum associated with the amplifier and includes a first rare earth-doped gain medium referred to as a first gain stage of the amplifier, wherein a filter is distributed over the first gain medium. The filter provides an attenuation of light associated with amplified spontaneous emission. The amplifier further includes a second rare earth-doped gain medium referred to as a second gain stage of the amplifier connected to the first gain stage; and a source of pump power connected to the amplifier for stimulating the rare earth-doped gain media. In an aspect of this embodiment, the second gain stage is preferably serially connected to the first gain stage closer to an output location of the first gain stage than to an input location in terms of signal propagation direction. In another aspect of this embodiment, a filter is also distributed over the second gain stage of the amplifier to further reduce ASE generated by the amplifier. In another aspect of this embodiment the pump source is preferably coupled to the first gain stage at a location closer to an input of the first gain stage than to an output of the first gain stage.
In a continuous distributed filtering aspect of both embodiments described above, the distributed filter is a rare earth doped, multiple and preferably dual core fiber making up the first gain stage. One of the cores is pumped to provide gain for the useful gain spectrum and the other core is unpumped causing it to absorb the out-of-band (ASE) light.
In another continuous distributed filtering aspect, the distributed filter is a non-adiabatically tapered fiber making up the first gain stage in which mode coupling occurs in the taper region to provide the filtering effect.
In a further continuous distributed filtering aspect, the distributed filter is a rare earth doped fiber making up the first gain stage and having a doped axial core and a doped or undoped coaxial annular core wherein bend loss provides the filtering effect over the length of the fiber.
In an alternative discrete filtering aspect, the distributed filter is a series of discrete filters such as long period gratings that are written or spliced into the rare earth doped fiber making up the first gain stage. In this aspect, it may be desirable to provide a doped fiber glass host different from a typical (germano)-alumino-silicate host glass, such as a phospho-silicate glass, that provides a more efficient medium for writing gratings therein.
The invention described herein particularly provides a device and a method for amplifying light signals in the erbium L-band having improved performance over L-band amplifiers without distributed filtering. Distributed filtering according to the invention substantially eliminates the out-of-band ASE generated in the amplifier due particularly to hard pumping, which in turn allows the amplifier to operate at a higher average inversion without self-saturation by the C-band ASE. Higher average inversion operation allows for a shorter active coil length for obtaining target gain values and in addition contributes to improved power conversion efficiency due to a reduction in background loss. Amplifier noise figure is also improved by the ability to achieve the target L-band stage gain at a higher inversion. The invention thus also provides benefits for amplifier circuit layout and packaging considerations.
Although the device according to the invention is illustratively described as a fiber optical amplifier, it is not so limited as a planar architecture, for example, can also implement the invention.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.